This study consists of a simulation and an experiment. The purpose of the simulation was to confirm that the individual differences in performance found in spaced learning using the Low-First method were mainly due to individual differences in working-memory capacity. The Low-First method is a spaced learning method which was derived from a reactivation theory of spacing effects in order to produce the largest possible spacing effects. The aims of the experiment were to improve the Low-First method by making it adaptive to the individual differences in working-memory capacity, and to further prove the effectiveness of the modified Low-First method. In the simulation, the learning processes of the participants with the three highest and the three lowest probabilities of recall were simulated using the reactivation model, changing only the activity decay speed. It was found that the estimates of probability of recall closely approximated the test results, indicating that individual differences in working-memory capacity were the primary cause of the disparities in probability of recall. The Low-First method was then modified by incorporating a new principle defining the error limit of a learning session to make it adaptive to the individual differences. In the experiment, two groups of participants with large and small working-memory capacities studied using either the original Low-First method or the modified Low-First method. The results showed that the modified Low-First method enhanced the effectiveness, time-efficiency, and smoothness of learning, especially for the participants with smaller working-memory capacities.